A conventional utility electricity parallel-connection system usually has an inverter circuit that can be controlled in a digital manner. The inverter circuit detects a utility electricity voltage to generate sinusoidal synchronous signals. The sinusoidal synchronous signals are sent to a current loop or AC voltage control loop in the inverter circuit so that the voltage, frequency and phase of the inverter circuit equal to the voltage, frequency and phase of the utility electricity, thereby achieving power flow control between the inverter circuit and the utility electricity.
A conventional inverter circuit 1000, as shown in FIG. 1, comprises a comparator or operational amplifier 1100 for detecting the zero-crossing point of utility electricity voltage VS. The zero-crossing signal SZERO not only calculates its frequency by digital capture 1200 but also uses the zero-crossing signal SZERO to reset a predetermined sine table 1300 and generate synchronous sinusoidal wave signal SSIN required for the inverter circuit 1000.
However, when the inverter circuit 1000 is operating, the reset signal is predisposed to oscillation, because of utility electricity voltage distortion and oscillation of the zero-crossing signal of the detection circuit, thereby causing sine table oscillation. In an attempt to solve the aforesaid problems, waveforms of the detected utility electricity voltage are improved with a low-pass filter, and signal oscillation is alleviated by a comparator with magnetic hysteresis; however, the techniques lead to phase delay of a sine table and therefore are inapplicable whenever voltage frequency variations are large.